1. Field of the Invention
The present invention relates to a transimpedance amplifier and an analog-digital converter circuit.
2. Description of Related Art
Up to now, in the case of inputting an analog signal from a high-voltage drive circuit to a low-voltage digital control circuit, the analog signal of the high voltage system is converted into a binary digital signal by a comparator or the like on the high voltage system side. After that, the digital signal is input to the digital control circuit of the low voltage system through isolation means such as a photocoupler. For this reason, only simple binary error signals indicating a voltage abnormality, for example, can be input to the digital control circuit.
In recent years, in order to achieve more sophisticated control, a technique has been developed in which an analog signal of a high voltage system is converted into a digital signal by an A/D converter on the low voltage system side through isolation means. FIG. 6 shows an optically isolated A/D converter circuit as described above. In the A/D converter circuit, a light-emitting diode (LED) 11 of a high voltage system and a photodiode 12 of a low voltage system are connected to each other through a photocoupler 13 serving as optical isolation means. A transimpedance AMP 14 converts a minute current (e.g., about 0 to 20 μA) from the photodiode 12 into a voltage and outputs the voltage to an A/D converter 15. The A/D converter 15 is, for example, a sequential comparison type A/D converter as disclosed in Japanese Unexamined Patent Application Publication No. 05-122076.
FIG. 7 is a block diagram showing the sequential comparison type A/D converter disclosed in FIG. 1 of Japanese Unexamined Patent Application Publication No. 05-122076. The A/D converter 15 includes a comparator 2, a D/A converter 3, a sequential comparison register 4, and an A/D converter control circuit 5. The operation of the A/D converter 15 will be described below.
The sequential comparison register 4 counts clocks supplied from the A/D converter control circuit 5, and outputs a digital signal DS1, which increases at a constant rate, to the D/A converter 3. The D/A converter 3 converts a digital signal DS2 into an analog voltage and outputs the analog voltage to the comparator 2. The comparator 2 compares a received analog input voltage with an output voltage of the D/A converter 3. When the analog input voltage and the output voltage of the D/A converter 3 are at the same voltage level, an output of the comparator 2 is inverted and the sequential comparison register 4 stops the counting operation. As a result, the voltage rise of the D/A converter 103 is stopped at a voltage equal to the analog input voltage. The value of the sequential comparison register 4 at this time is equal to a value obtained by digital conversion of the analog input voltage, and serves as the output of the A/D converter.
In this case, the photodiode 12, the transimpedance AMP 14, and the A/D converter 15, which are shown in FIG. 6, are integrated into one chip as a light receiving IC for a photocoupler. This leads to great improvements in convenience and reduction in size of a mounting space.